Runtime

Every Java application has a single instance of class
Runtime that allows the application to interface with
the environment in which the application is running. The current
runtime can be obtained from the getRuntime method.

This method was deprecated
in API level 1.
As of JDK 1.1, the preferred way to translate a byte
stream in the local encoding into a character stream in Unicode is via
the InputStreamReader and BufferedReader
classes.

This method was deprecated
in API level 1.
As of JDK 1.1, the preferred way to translate a
Unicode character stream into a byte stream in the local encoding is via
the OutputStreamWriter, BufferedWriter, and
PrintWriter classes.

This method was deprecated
in API level 1.
This method is inherently unsafe. It may result in
finalizers being called on live objects while other threads are
concurrently manipulating those objects, resulting in erratic
behavior or deadlock.

Causes the current thread to wait until another thread invokes the
notify() method or the
notifyAll() method for this object, or
some other thread interrupts the current thread, or a certain
amount of real time has elapsed.

Public methods

addShutdownHook

The Java virtual machine shuts down in response to two kinds
of events:

The program exits normally, when the last non-daemon
thread exits or when the exit (equivalently,
System.exit) method is invoked, or

The virtual machine is terminated in response to a
user interrupt, such as typing ^C, or a system-wide event,
such as user logoff or system shutdown.

A shutdown hook is simply an initialized but unstarted
thread. When the virtual machine begins its shutdown sequence it will
start all registered shutdown hooks in some unspecified order and let
them run concurrently. When all the hooks have finished it will then
run all uninvoked finalizers if finalization-on-exit has been enabled.
Finally, the virtual machine will halt. Note that daemon threads will
continue to run during the shutdown sequence, as will non-daemon threads
if shutdown was initiated by invoking the exit
method.

Once the shutdown sequence has begun it can be stopped only by
invoking the halt method, which forcibly
terminates the virtual machine.

Once the shutdown sequence has begun it is impossible to register a
new shutdown hook or de-register a previously-registered hook.
Attempting either of these operations will cause an
IllegalStateException to be thrown.

Shutdown hooks run at a delicate time in the life cycle of a virtual
machine and should therefore be coded defensively. They should, in
particular, be written to be thread-safe and to avoid deadlocks insofar
as possible. They should also not rely blindly upon services that may
have registered their own shutdown hooks and therefore may themselves in
the process of shutting down. Attempts to use other thread-based
services such as the AWT event-dispatch thread, for example, may lead to
deadlocks.

Shutdown hooks should also finish their work quickly. When a
program invokes exit the expectation is
that the virtual machine will promptly shut down and exit. When the
virtual machine is terminated due to user logoff or system shutdown the
underlying operating system may only allow a fixed amount of time in
which to shut down and exit. It is therefore inadvisable to attempt any
user interaction or to perform a long-running computation in a shutdown
hook.

Uncaught exceptions are handled in shutdown hooks just as in any
other thread, by invoking the uncaughtException method of the thread's ThreadGroup object. The default implementation of this method
prints the exception's stack trace to err and
terminates the thread; it does not cause the virtual machine to exit or
halt.

In rare circumstances the virtual machine may abort, that is,
stop running without shutting down cleanly. This occurs when the
virtual machine is terminated externally, for example with the
SIGKILL signal on Unix or the TerminateProcess call on
Microsoft Windows. The virtual machine may also abort if a native
method goes awry by, for example, corrupting internal data structures or
attempting to access nonexistent memory. If the virtual machine aborts
then no guarantee can be made about whether or not any shutdown hooks
will be run.

availableProcessors

Returns the number of processors available to the Java virtual machine.

This value may change during a particular invocation of the virtual
machine. Applications that are sensitive to the number of available
processors should therefore occasionally poll this property and adjust
their resource usage appropriately.

Returns

int

the maximum number of processors available to the virtual
machine; never smaller than one

exec

Executes the specified command and arguments in a separate process with
the specified environment and working directory.

Given an array of strings cmdarray, representing the
tokens of a command line, and an array of strings envp,
representing "environment" variable settings, this method creates
a new process in which to execute the specified command.

This method checks that cmdarray is a valid operating
system command. Which commands are valid is system-dependent,
but at the very least the command must be a non-empty list of
non-null strings.

If envp is null, the subprocess inherits the
environment settings of the current process.

A minimal set of system dependent environment variables may
be required to start a process on some operating systems.
As a result, the subprocess may inherit additional environment variable
settings beyond those in the specified environment.

start() is now the preferred way to
start a process with a modified environment.

The working directory of the new subprocess is specified by dir.
If dir is null, the subprocess inherits the
current working directory of the current process.

If a security manager exists, its
checkExec
method is invoked with the first component of the array
cmdarray as its argument. This may result in a
SecurityException being thrown.

Starting an operating system process is highly system-dependent.
Among the many things that can go wrong are:

The operating system program file was not found.

Access to the program file was denied.

The working directory does not exist.

In such cases an exception will be thrown. The exact nature
of the exception is system-dependent, but it will always be a
subclass of IOException.

Parameters

cmdarray

String: array containing the command to call and
its arguments.

envp

String: array of strings, each element of which
has environment variable settings in the format
name=value, or
null if the subprocess should inherit
the environment of the current process.

dir

File: the working directory of the subprocess, or
null if the subprocess should inherit
the working directory of the current process.

exec

Executes the specified string command in a separate process with the
specified environment and working directory.

This is a convenience method. An invocation of the form
exec(command, envp, dir)
behaves in exactly the same way as the invocation
exec(cmdarray, envp, dir),
where cmdarray is an array of all the tokens in
command.

More precisely, the command string is broken
into tokens using a StringTokenizer created by the call
new StringTokenizer(command) with no
further modification of the character categories. The tokens
produced by the tokenizer are then placed in the new string
array cmdarray, in the same order.

Parameters

command

String: a specified system command.

envp

String: array of strings, each element of which
has environment variable settings in the format
name=value, or
null if the subprocess should inherit
the environment of the current process.

dir

File: the working directory of the subprocess, or
null if the subprocess should inherit
the working directory of the current process.

exit

Terminates the currently running Java virtual machine by initiating its
shutdown sequence. This method never returns normally. The argument
serves as a status code; by convention, a nonzero status code indicates
abnormal termination.

The virtual machine's shutdown sequence consists of two phases. In
the first phase all registered shutdown hooks,
if any, are started in some unspecified order and allowed to run
concurrently until they finish. In the second phase all uninvoked
finalizers are run if finalization-on-exit
has been enabled. Once this is done the virtual machine halts.

If this method is invoked after the virtual machine has begun its
shutdown sequence then if shutdown hooks are being run this method will
block indefinitely. If shutdown hooks have already been run and on-exit
finalization has been enabled then this method halts the virtual machine
with the given status code if the status is nonzero; otherwise, it
blocks indefinitely.

The System.exit method is the
conventional and convenient means of invoking this method.

gc

Runs the garbage collector.
Calling this method suggests that the Java virtual machine expend
effort toward recycling unused objects in order to make the memory
they currently occupy available for quick reuse. When control
returns from the method call, the virtual machine has made
its best effort to recycle all discarded objects.

The name gc stands for "garbage
collector". The virtual machine performs this recycling
process automatically as needed, in a separate thread, even if the
gc method is not invoked explicitly.

The method gc() is the conventional and convenient
means of invoking this method.

getLocalizedInputStream

This method was deprecated
in API level 1.
As of JDK 1.1, the preferred way to translate a byte
stream in the local encoding into a character stream in Unicode is via
the InputStreamReader and BufferedReader
classes.

Creates a localized version of an input stream. This method takes
an InputStream and returns an InputStream
equivalent to the argument in all respects except that it is
localized: as characters in the local character set are read from
the stream, they are automatically converted from the local
character set to Unicode.

If the argument is already a localized stream, it may be returned
as the result.

getLocalizedOutputStream

This method was deprecated
in API level 1.
As of JDK 1.1, the preferred way to translate a
Unicode character stream into a byte stream in the local encoding is via
the OutputStreamWriter, BufferedWriter, and
PrintWriter classes.

Creates a localized version of an output stream. This method
takes an OutputStream and returns an
OutputStream equivalent to the argument in all respects
except that it is localized: as Unicode characters are written to
the stream, they are automatically converted to the local
character set.

If the argument is already a localized stream, it may be returned
as the result.

halt

Forcibly terminates the currently running Java virtual machine. This
method never returns normally.

This method should be used with extreme caution. Unlike the
exit method, this method does not cause shutdown
hooks to be started and does not run uninvoked finalizers if
finalization-on-exit has been enabled. If the shutdown sequence has
already been initiated then this method does not wait for any running
shutdown hooks or finalizers to finish their work.

Parameters

status

int: Termination status. By convention, a nonzero status code
indicates abnormal termination. If the exit (equivalently, System.exit) method has already been invoked then this
status code will override the status code passed to that method.

load

Loads the native library specified by the filename argument. The filename
argument must be an absolute path name.
(for example
Runtime.getRuntime().load("/home/avh/lib/libX11.so");).
If the filename argument, when stripped of any platform-specific library
prefix, path, and file extension, indicates a library whose name is,
for example, L, and a native library called L is statically linked
with the VM, then the JNI_OnLoad_L function exported by the library
is invoked rather than attempting to load a dynamic library.
A filename matching the argument does not have to exist in the file
system. See the JNI Specification for more details.
Otherwise, the filename argument is mapped to a native library image in
an implementation-dependent manner.

First, if there is a security manager, its checkLink
method is called with the filename as its argument.
This may result in a security exception.

This is similar to the method loadLibrary(String), but it
accepts a general file name as an argument rather than just a library
name, allowing any file of native code to be loaded.

The method load(String) is the conventional and
convenient means of invoking this method.

loadLibrary

Loads the native library specified by the libname
argument. The libname argument must not contain any platform
specific prefix, file extension or path. If a native library
called libname is statically linked with the VM, then the
JNI_OnLoad_libname function exported by the library is invoked.
See the JNI Specification for more details.
Otherwise, the libname argument is loaded from a system library
location and mapped to a native library image in an implementation-
dependent manner.

First, if there is a security manager, its checkLink
method is called with the libname as its argument.
This may result in a security exception.

The method loadLibrary(String) is the conventional
and convenient means of invoking this method. If native
methods are to be used in the implementation of a class, a standard
strategy is to put the native code in a library file (call it
LibFile) and then to put a static initializer:

static { System.loadLibrary("LibFile"); }

within the class declaration. When the class is loaded and
initialized, the necessary native code implementation for the native
methods will then be loaded as well.

If this method is called more than once with the same library
name, the second and subsequent calls are ignored.

runFinalization

Runs the finalization methods of any objects pending finalization.
Calling this method suggests that the Java virtual machine expend
effort toward running the finalize methods of objects
that have been found to be discarded but whose finalize
methods have not yet been run. When control returns from the
method call, the virtual machine has made a best effort to
complete all outstanding finalizations.

The virtual machine performs the finalization process
automatically as needed, in a separate thread, if the
runFinalization method is not invoked explicitly.

The method runFinalization() is the conventional
and convenient means of invoking this method.

runFinalizersOnExit

This method was deprecated
in API level 1.
This method is inherently unsafe. It may result in
finalizers being called on live objects while other threads are
concurrently manipulating those objects, resulting in erratic
behavior or deadlock.

Enable or disable finalization on exit; doing so specifies that the
finalizers of all objects that have finalizers that have not yet been
automatically invoked are to be run before the Java runtime exits.
By default, finalization on exit is disabled.

If there is a security manager,
its checkExit method is first called
with 0 as its argument to ensure the exit is allowed.
This could result in a SecurityException.

traceInstructions

Enables/Disables tracing of instructions.
If the boolean argument is true, this
method suggests that the Java virtual machine emit debugging
information for each instruction in the virtual machine as it
is executed. The format of this information, and the file or other
output stream to which it is emitted, depends on the host environment.
The virtual machine may ignore this request if it does not support
this feature. The destination of the trace output is system
dependent.

If the boolean argument is false, this
method causes the virtual machine to stop performing the
detailed instruction trace it is performing.

traceMethodCalls

Enables/Disables tracing of method calls.
If the boolean argument is true, this
method suggests that the Java virtual machine emit debugging
information for each method in the virtual machine as it is
called. The format of this information, and the file or other output
stream to which it is emitted, depends on the host environment. The
virtual machine may ignore this request if it does not support
this feature.